Power and free conveyor system

ABSTRACT

A power-and-free conveyor system of the type having work carriers each consisting of coupled front and rear trolleys adapted to be propelled along a track by lugs on an overhead chain drive and each adapted further to be disconnected from the drive in a localized close pack section of the conveyor by a cam plate on the rear trolley of the preceding carrier, and wherein the drive and the cam plate of each carrier have novel means which are uniquely cooperative to prevent the carrier from backsliding on inclined portions of the track in the event the front trolley thereof inadvertently becomes uncoupled from the drive and also to more effectively and efficiently transfer the carrier through a switch point from one conveyor to another or from a conveyor onto a spur track for storage or the like.

United States Patent 1 June 6, 1972 Czarnecki 41 POWER AND FREE CONVEYOR SYSTEM [72] Inventor: Adolph Czarnecki, Birmingham, Mich.

[73] Assignee: Standard Alliance Industries, Inc.,

Chicago, Ill.

[22] Filed: Sept. 3, 1969 21 Appl.N0.: 854,817

[52] U.S.Cl. ....l04/172S [51] lnt.Cl ..B65g 17/42 [58] FieldofSearch ..104/l72,l78

[56] References Cited UNITED STATES PATENTS 2,956,51410/1960 Bishop ..I04/128 3,247,806 4/1966 Dehne..... ....lO4/l72 3,415,201 12/1968 Czarnecki "104/178 3.523.504 8/1970 Jones ..104/172 3.31 4,377 4/1967 Dehne ..l04/96 Primary Examiner--Anhur L. La Point Assistant Examiner-Robert W. Saifer An0rneyl-lamess, Dickey & Pierce [57] ABSTRACT A power-and-free conveyor system of the type having work carriers each consisting of coupled front and rear trolleys adapted to be propelled along a track by lugs-on an overhead chain drive and each adapted further to be disconnected from the drive in a localized close pack section of the conveyor by a cam plate on the rear trolley of the preceding carrier, and wherein the drive and the cam plate of each carrier have novel means which are uniquely cooperative to prevent the carrier from back-sliding on inclined portions of the track in the event the front trolley thereof inadvertently becomes uncoupled from the drive and also to more effectively and efficiently transfer the carrier through a switch point from one conveyor to another or from a conveyor onto a spur track for storage or the like.

7 Claims, 7 Drawing Figures PATENTEDJUH 61972 3.667, 399 sum 30F 3 IN VENTOR.

BACKGROUND OF THE INVENTION Specifically, the present invention is an improvement in the close pack power-and-free conveyor system disclosed in the Czarnecki US. Pat. No. 3,415,201 which issued Dec. 10, 1968. The patented system has work carriers of the type described above wherein the. front trolley has opposed, pivoted flop dogs which trap a pivoted cam dog or lug depending from the overhead drive chain to propel the carrier along the track and wherein the cam plate on the rear trolley is uniquely formed to disconnect a following carrier automatically from the power drive in selected close pack sections of the conveyor. In this system the pivoted drive lugs of the power chain permit disengagement of the carriers in the close pack sections but difficulty has been experienced on occasion with the lugs inadvertently disengaging their trolleys particularly on inclined sections of the track. Also, some difficulty has been experienced in special situations in moving the carriers through a switch onto'a second conveyor or a spur track. The usual practice has been to push the front trolley into the switch by the pivoted drive lug and then, after the lug wipes off sideways at the switch, engage the cam plate on the rear trolley by a second lug which pushes the carrier farther through the switch and onto the spur track or until the front trolley thereof is moved onto the track of the second conveyor far enough for engagement thereof by the power drive of that conveyor. This arrangement, however, was not entirely satisfactoryas it required additional length of conveyor and expensive fabrication of track in switch areas. Also, it required precise spacing between the drive and conveyor tracks which is difficult to achieve and to maintain in equipment of this type, and it further required an abrupt change in the spacing of the tracks at the switch point which complicated installation and operation of the system.

SUMMARY OF THE INVENTION The above difficulties are overcome by the present invention by providing the drive chain with a combination of spaced pivoted and fixed driving lugs and by providing an upstanding abutment on the cam plate of the rear trolley which, under normal operating conditions, is engageable by the fixed lugs only of the drive chain. In practice, the pivoted and fixed lugs of the drive chain are arranged alternately in pairs and the two lugs of each pair are spaced farther apart than the front and rear trolleys of the work carriers so that the fixed lug normally is spaced behind the rear trolley of the carrier. Then, in the event the carrier becomes disengaged from the pivoted lug of the drive chain through inadvertence, or otherwise, the fixed lug quickly catches up to the rear trolley and pushes the canier forward. As suggested, inadvertent disengagement between the drive and the carrier occurs usually on inclined sections of the conveyor; and, under the circumstances recited above, the fixed lug simply engages the abutment preventing the carrier from back-sliding and continuing to push it to the top of the incline. At a switch point where the carrier is to be moved off of the conveyor onto another conveyor or onto a spur track,

the fixed lug engages the abutment after the pivoted lug has wiped off laterally and disengaged the flop dog of the front trolley and pushes the carrier ahead through the switch and onto the other track. This arrangement does not require close tolerance spacing between the drive and carrier tracks nor does it require any variation at the switch area from the normal structure of the conveyor system. In close pack sections of the conveyor, the spacing between the drive and carrier tracks is increased slightly to provide vertical clearance between the fixed lugs and the abutments so that close packing can occur normally as disclosed in the Czarnecki patent identified above. However, precise, close tolerance spacing between the tracks does not have to be maintained even under these circumstances and the location of the point at which the change in spacing occurs is not critical.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view showing a portion of a conveyor system embodying the invention and illustrating the drive and carrier tracks spaced relatively close together for normal operation;

FIG, 2 is an enlarged, fragmentary, horizontal sectional view taken on the line 22 of FIG. 1;

FIG. 3 is a vertical sectional view taken on the line 3-3 of FIG. I

FIG. 4 is a top plan view of the cam plate on the rear trolley looking in the direction of the arrow 4 in FIG. 1;

FIG. 5 is an enlarged, fragmentary, side elevational view spaced relatively farther apart for close pack operation;

FIG. 6 is a vertical sectional view taken on the line 6-6 of FIG. 5; and I FIG. 7 is a fragmentary, diagrammatic view showing a typical switch point in a conveyor system at which trolleys can be switched away from the conveyor and onto a spur track.

DESCRIPTION OF THE PREFERRED EMBODIMENT Reference is first had to FIG. 1 which shows a typical section in side elevation of a conveyor system embodying the invention. This is an overhead power and free, closed loop type of conveyor system which is used primarily to carry goods and articles of manufacture, assembly-line fashion, from place to place in a factory or the like. These conveyors frequently are exceedingly long and, where the installation is very large, a plurality of individually operated but mutually coordinated conveyors are sometimes used together in a macrosystem. Switch points may be provided between conveyors for transferring work carriers from one conveyor to the other so that workpieces can be transported without interruption and often automatically throughout the entire system. The different conveyors can be operated at the same speed or at different speeds, and they areused for an almost infinite number of purposes; as, for example, in'assembly lines, for the delivery of parts to assembly lines, for the movement of parts, subassemblies and assemblies through cleaning baths, paint spray booths and drying ovens, and for the delivery of finished goods to or from shipping docks and storageareas.

A typical conveyor comprises spaced, parallel, upper and lower drive and carrier tracks 10 and 12, respectively. In the particular form of invention here shown, the drive track 10 is in the form of an I-beam l4; and the carrier track 12 comprises a pair of laterally spaced inwardly facing channels 16 and 18 (FIG. 6). Open frame members 20 of inverted U-shape disposed at spaced intervals along the tracks 10 and 12 are welded or otherwise attached to the I-beam l4 and to the channels 16 and 18, as shown in FIG. 6, to hold the track members rigidly spaced and properly oriented with respect to each other; and the frames 20, in turn, are attached in any conventional manner to suitable overhead supporting structure (not shown).

Work carriers 22 mounted to move on and along the track 12 comprise front and rear wheeled trolleys 24 and 26 of a type shown generally in the Czarnecki US. Pat. No. 3,415,201 referred to above and the disclosure of that patent insofar as it is applicable is incorporated by reference herein. A brief description of the carrier 22 therefor will suffice.

More particularly, the front trolley 24 has a vertical, platelike body 28 suspended between front and rear pairs of wheels 30 and 31. The two front wheels 30 are mounted for rotation on a front axle 32 carried by the body 28, and they are 7 shown in FIG. 6. In this connection, it will be appreciated that the other pair of wheels 31 are similarly mounted for rotation on a rear axle 33 and at opposite sides of the body 28 for travel in the channels 16 and 18. Opposed flop dogs 34 and 35 are mounted in the middle of the body 28 on pivots 36 and 38,

respectively. The upper portions of the flop dogs 34 and 35 extend upwardly between the channels 16 and 18, and the lower portions thereof are counterweighted, as at 39 and 40, to hold the flop dogs normally upright, as shown in FIG. 5. Both of the flop dogs 34 and 35 are free to pivot inwardly; viz, each towardthe other, but the counterweighted lower ends butt against the body 28 to prevent counter-rotational or pivotal movement thereof from the normal upright position. Front and rear horizontal rollers 41 42 disposed in slots 43 and 44 provided in the body 28 are mounted for rotation on journals 45 and 46 carried by the body. As shown, these horizontal rollers 41. and 42 are disposed directly between the lower flanges of the channels 16 and 18, and they contact and roll on the edges of the flanges to hold the trolley 24 centered with respect to the channels and to guide the same during movement thereof along the carrier track 12.

The rear trolley 26 comprises a body 47 which is similar in all respects to the body 28 of the front trolley 24; and it is similarly provided with front and rear pairs of wheels 48 and 49. The two front wheels 48 are joumaled for rotation on an axle 50 carried by and extending transversely of the body 47; and the two rear wheels 49 are rotatably mounted on an axle 51 also carried by the body. Both pairs of wheels 48 and 49 travel on the lower flanges of the channels 16 and 18 with the body 47 suspended between and extending below the channels. Front 'and rear guide rollers 52 and 53 disposed in forwardly and rearwardly facing slots 54 and 55 provided in the body '47 and rotatably mounted on journals 56 and 57 travel on and between the lower flanges of the channels 16 and 18 to hold the rear trolley 26 centered between the channels and to guide the same for movement along the carrier track 12. Instead of flop dogs, however, the body 47 of the rear trolley 26 carries a cam plate 58 having a generally vertical middle web portion 59 disposed between the upper flanges of the channels '16 and 18 and'attached to the body 47 by cross pins 60 and 61. The cam plate 58 is supported entirely above the channels 16 and 18 by the web portion 59, and with the flat top surface 62 thereof disposed at the same level as the tips of the flop dogs 34 and 35. Front and rear ramps 63 and 64 formed on the cam plate 58 at opposite ends of the top surface 62 extend angularly downwardly from the topsurface. A rearwardly opening medial slot 66 in the cam plate 58 is adapted to receive the front flop dog 34 of a following work carrier in a manner hereinafter described in detail. I

- Swivelmountings 68 and 70 attached to the trolley bodies 28 and 47, respectively, by horizontal pivots 72 and 74 are coupled together by a crossbar 76 which is attached to the mountings by vertical pivots 78 and 80. A suitable hanger 82 attached to the crossbar 76 substantially midway between the front and rear trolleys 24 and 26 is adapted to carry workpieces or the like. These hangers are conventional and vary considerably in form depending uponthe kind and size of workpieces being transported by the conveyor.

Suspended from the I-beam 14 of the drive track by lontional type having pairs of links 88 pivotally interconnected by single links 90. Attached to and depending from the chain 86 at spaced intervals therealong are alternately pivoted and fixed drive lugs 92 and 94, respectively.

The upper end of each of the pivoted lugs 92 is disposed between laterally spaced mounting brackets 96 on and depending from one of the bottom links 88 for free pivotal movement on a cross pin 98 which extends between and is carried bythe brackets. The lower end 100 of the pivoted lug 92 is disposed in the path of travel of the flop dogs 34 and 35, and it is relatively wide to assure sustained engagement thereof with the flop dogs.'The chain 86 is driven in the direction of the arrow 101 by a rotatably driven sprocket (not shown) according to conventional practice and, it will be observed that the lug 92 extends downwardly and rearwardly from the chain.

The tip 102 of the lug 92 is engageable with the link 88 on which it is mounted to limit downward movement of the lower end 100 and to hold the lug at all times in the downwardly and gitudinally spaced trolleys 84 is a drive chain 86 of a convenv arm portion 106 depends from the plate and tenninates at the lower end thereof in a relatively wide horizontal pusher plate 108 which is positioned to engage an upstanding abutment 1 10 on the cam plate of the rear trolley 26. As shown in FIG. 4, the abutment l 10 is mounted in the middle of the cam plate 58 directly in front of the slot 66, and the rearwardly facing edge 112 of the abutment is convexly curved. The forward edge 114 of the plate 108 is similarly curved convexly, as shown in FIG. 2.

In practice, the conveyor is installed with the drive track 10 above and parallel to the carrier track 12 and, for normal operation, with the two tracks spaced apart so that the pusher plate 108 of the fixed lug 94 will engage the abutment 110. In other words, the pusher plate 108 should be above or it should at least clear the top surface 62 of the cam plate 58 but be low enough so that it will not clear the top of the abutment l 10. The abutment can be of any suitable height that will permit this relationship to be maintained without the necessity of holding the spatial relationship between the two drives 10 and 12 to a close tolerance dimension so as to unduly complicate and add to the expense of the conveyor installation and maintenance. In general, workmen can easily hold a dimensionof v at least l inch, or perhaps a little less, and an abutment heightof 1% inch has been found to be satisfactory.

As suggested, the work carriers 22 normally are pushed along the track 12 by the pivoted lugs 92 each of which engages the front flop dog 34 of its respective carrier, as shown in FIG. 5. In this connection, it will be observed that the lower end of the lug 92 is inclined forwardly substantially at right angles to the main shank portion of the lug and that the tip of the flop dog 34 is undercut, as at 116, at the inner side thereof so that the dog-engaging-end of the lug'and the tip of the dog under and overlap each other to assure-proper engagement and adequate purchase therebetween even though the angular position of the lug may vary somewhat from time to time as by variation in the spacing between the tracks 10 and 12. This spacing usually varies somewhat along the length of the conveyor within permissible tolerance amounts normal for equipment of this type. Indeed, the spacing may be deliberately varied in different localized sections of the conveyor, as for example, at drying ovens or in other areas where the work carriers 22 are close packed. I

In the event an open lug 92 overtakes a stationary work carrier 22 as happens for example if a carrier is pushed onto the track 12 through a switch point at an open spot in the drive 10, it will engage automatically with the leading flop dog 34 of the front trolley 24. If the open lug 92 happens to be behind the rear trolley 26 at this time, it will simply override the cam plate 58 and the abutment 110. Although an open lug 92 will hang downwardly considerably farther than shown in the drawing, the lower end 100 thereof simply rides up on the ramp 64 of the cam plate 58. When it reaches the abutment 1 10, a center recess 118 in the end 100 and a beveled surface 120 at the leading end of therecess causes the end 100 to ride also over the abutment. Thus, the pivoted lug 92 will not engage and drive the work carrier 22 through the cam plate 58. As the lug 92 moves forwardly past the abutment 110, the lower end 100 simply rides downwardly on the front ramp 63 of the cam plate and it therefor continues to advance without effect as far as the work carrier is concerned until it strikes the rear flop dog 35. When the lug 92 engages the beveled rear surface 122, it causes the flop dog 35 to tip forwardly so that the lug can enter the space between the two flop dogs 34 and 35. As soon as the lug 92 overrides the rear flop dog 35, the counterweighted lower end 40 of the latter causes it immediately to resume its normal upright position. However,

when the lug 92 engages the front flop dog 34 it immediately hooks onto the undercut tip thereof; and, as it does so, the lug switches to whatever angle is required to effect a solid mating engagement with the flop dog. This action effectively couples the work carrier 22 to the drive chain 86 and causes the carrier to proceed along the conveyor until it is disengaged in some manner from the drive.

As suggested, the work carrier 22 sometimes becomes disengaged from the drive chain 86 through inadvertence. This, of course, does not happen very often; but, when it does happen, it usually occurs while the carrier is traversing an incline. This normally results in the carrier 22 back-sliding into the following carrier; and this in turn causes a jam-up. The arrangement of alternate pivoted and fixed lugs 92 and 94 in combination with the abutment 110 on the cam plate 58 of the rear trolley 26, however, prevents this kind of occurrence. If a work carrier 22 gets loose from the drive chain 86 under these circumstances, the following fixed lug 94 immediately catches the abutment 110 'and pushes the work carrier on up the incline. It then continues to push the carrier 22 until it can be brought again into engagement with the pivoted lug 92 as by a workman pushing the carrier forwardly to re-engage the front trolley 24 with the lug. In this connection, it will be observed that the leading flop dog 34 also has a beveled front face 124 which causes the front dog of an overtaking carrier 22 to duck under the dog-engaging-end 100 of the pivoted lug 92 and effect re-engagement of the lug with the trolley 24. A work carrier 22 traversing a decline in the conveyor cannot override the pivoted lug 92 as the end 100 thereof is trapped between the two dogs 34 and 35. If the canier 22 should tend to coast down the incline faster than the rate of movement of the drive chain 86, the vertical inner face 126 of the rear flop dog 35 butts against the vertical rear face 128 of the lug 92 to prevent further overriding travel of the carrier.

When it is desired to disengage the work carrier 22' deliberately from the drive chain 86, as for example in a close pack section of the conveyor, the spacing between the drive track and the carrier track 12 is deliberately increased sufficiently so that the fixed lugs 94 clear not only the top surfaces 62 of the cam plates 58 but also the abutments 110. Under the circumstances and conditions previously given, an increase of it of an inch is adequate for this purpose. This in crease in the spacing between the two tracks 10 and 12 in no way affects the connections between the pivoted lugs 92 and the front trolleys 24, however, as the lugs 92 simply adjust their angular position to accommodate the difference in spacing between the tracks. Thus, the spacing change can be made at any convenient location and the particular point of change is not critical in any way to the operation of the conveyor. In close pack operation, suitable means conventionally is provided for disconnecting the pivoted lug 92 from the first work carrier to enter the close pack section; and thereafter each carrier to arrive at the close pack section pushes the carrier or carriers ahead of it in the section. Carriers emerging from the far side of the close pack section are re-engaged automatically by pivoted lugs 92 on the drive chain 86. More particularly, a work carrier 22 that has been disengaged from the drive chain 86 at the entrance of the close pack section simply sits there until the following carrier bumps into it from behind. What happens then is that the lead trolley 24 of the following carrier enters the rearwardly facing slot 66 in the cam plate of the disengaged carrier. As the front flop dog 34 of the following carrier moves into the slot 66, the lower end 100 of the coacting driving lug 92 rides up on the ramp 64 which causes the lug 92 to move upwardly out of engagement with the dog. After disengagement, momentum carries the carrier 22 forwardly until the dog 34 is at the extreme inner end of the slot 66. Manifestly, the lug 92 will not thereafter engage any front trolley 24 which is nested in the cam plate of a preceding carrier, and the lug simply ratchets freely along the close packed carriers. At the discharge end of a close pack section, the drive and carrier tracks 10 and 12 are brought back into normal spatial relationship and each carrier emerging from the section is automatically picked up by the first pivoted lug 92 that happens along and is pushed thereby along the conveyor in normally spaced fashion.

Reference is now had to FIG. 7 which shows a typical switch point arrangement which utilizes to advantage the alternate pivoted and fixed lug drive arrangement in combination with the cam plate abutment 110. In FIG. 7, the numeral 12 diagrammatically designates the carrier track and the numeral 10 diagrammatically designates the drive track. The numeral 130 designates a spur track similar to the carrier track 12, although it could just as well be a second conveyor similar to the one hereinabove described. Suitable means may be provided for moving carriers under power along a spur track. In a typical situation, however, spur tracks are used for storing work carriers 22 and these tracks usually simply incline downwardly from the power track at an angle which causes carriers pushed thereon to slide along by gravity. The numeral 132 diagrammatically designates a conveyor switch of a conventional type which is pivoted at 134 for movement between the full line and broken line positions shown in the drawing. In the full line position, the switch 132 causes work carriers 22 to move along the track 12 without interruption. In the broken line position, the switch 132 causes work carriers 22 to move from the main track 12 onto the spur track 130. The switch 132 can be moved between the full and broken line positions either manually or by suitable conventional automatic means.

In utilizing the present novel arrangement at a switch point, such as the one shown in FIG. 7, the normal spatial relationship is maintained between the drive track 10 and the carrier track 12. If the switch 132 is in the broken line position (FIG. 7), the leading trolley 24 enters the switch and the lug 92 engaged therewith wipes ofi laterally as the drive chain 86 traverses the bend or curve in the main conveyor. The relatively wide lower end of the lug maintains contact with the dog 34 long enough under-these conditions to push the trolley 24 well into the switch. However, when the lug 92 comes free of the trolley 24, the carrier 22 may coast for a short distance but, for the most part, it simply sits until the following fixed lug 94 engages the abutment on the rear trolley 26 which, of course, is still on the main track 12. When this happens, the fixed lug 94 pushes the rear trolley 26 to advance the carrier through the switch 132 and onto the spur track 130. Here again, the relatively wide face 114 on the pusher plate 108 of the fixed lug 94 maintains contact with the abutment 110 long enough to push the rear trolley 26 well into the switch 132. In any event, by this time most of the work carrier 22 is on the inclined spur track and, as the fixed lug 94 wipes off of the abutment 110, it gives it a final push which together with the momentum of the moving carrier, causes it to move entirely through the switch and onto the spur track 130 where it coasts along by gravity to the lower end of the line or until it bumps into a preceding carrier.

It may thus be seen that the arrangement of alternate pivoted and fixed lugs in combination with the abutment 110 on the rear trolley of a work carrier 22 permits an exceedingly versatile operation of the conveyor. It eliminates problems heretofore inherent in conveyor systems of the type involved here, and it corrects these problems without unduly complicating either the installation or the maintenance of the conveyor.

What is claimed is:

1. In a power and free conveyor system a trolley track,

load carriers comprising coupled lead and follower trolleys movable on and along said trolley track,

the lead trolley of each carrier being equipped with at least one flop dog and the follower trolley thereof being equipped with a cam plate having a fixed abutment extending upwardly therefrom,

a chain track spaced above said trolley track, 'a drive chain suspended from and movable along said chain track equipped with longitudinally spaced pairs of depending lugs,

one lug of each pair being pivoted and the other lug of each pair being fixed, the pivoted lug of each pair adapted to engage the flop dog of a load carrier to drive the latter, and the fixed lug of said pair being normally spaced longitudinally behind the abutment of the ,cam plate on the follower trolley of said carrier but adapted to engage and to push against said abutment upon disengagement of said flop dog by said pivotedlug on occasions as by inadvertence or by movement of said lead trolley through a switch onto a second or spur trolley track, the vertical spacing between said trolley track and said chain track normally pennitting butting engagement between said pivoted lug and said flop dog and between said fixed lug and said abutment, but said tracks adapted to be spaced relatively farther apart in localized sections of the conveyor system as for close packing whereby to position said fixed lug at a level above said abutment to prevent relative engagement therebetween without affecting engagability of said pivoted lug with said flop dog. 2. The power and free conveyor system set forth in claim 1 wherein the cam plate of each load carrier has a rearwardly opening slot and a ramp at the open end of said slot, and

wherein the pivoted lugs extend downwardly and rearwardly from said drive chain, the flop dog engaging ends of said pivoted lugs are relatively wider than said flop dogs, the flop dog of each load carrier is adapted to enter the slot in the cam plate of a preceding load carrier and the ramp on the cam plate of said preceding load carrier is engageable by entry of said flop dog with the flop'dog-engagingend of the pivoted lug associated with said flop dog, whereby to disengage said lug from said flop dog. 3. The power and free conveyor system as set forth in claim 2 wherein said flop dogs and said pivoted lugs have terminal portions disposed to over-and under-lap each other when said drive lugs are in engagement with said flop dogs. 4. The power and free conveyor system set forth in claim 1 wherein said fixed lugs and said abutrnents have mutually engageable convex surfaces. 5. The power and free conveyor system set forth in claim 4 wherein one of said mutually engageable convex surfaces is relatively substantially wider than the other. 6. The power and free conveyor system set forth in claim 4 wherein the convex surfaces of said fixed lugs are relatively substantially wider than the convex surfaces of said abutments. 7. A power and free conveyor system having movable work carriers provided with coupled front and rear trolley members,

and associated power means having a plurality of pairs of spaced drive lugs,

one drive lug of each pair being relatively long and adapted to cooperate with said front trolley to move said work carrier, and the other drive lug of each pair being relatively short and adapted to cooperate with the rear trolley to move said carrier, one portion of said power means being spaced relatively close to said work carriers and another portion of said power means being spaced relatively far from said work carriers, said other relatively short drive lug being disposed to drivingly engage said rear trolley at said one portion of said power means and disposed to be not engageable' with said rear trolley at said other portion of said power means, said one relatively long drive lug being disposed to drivingly engage said front trolley at both portions of said power means and being provided with mounting means which is operative to adjust said lug automatically to compensate for variations in the relative spacing between said power means and said work carriers and to maintain said lug positioned for. operative engagement'with said work carriers under all such variations in spacing. 

1. In a power and free conveyor system a trolley track, load carriers comprising coupled lead and follower trolleys movable on and along said trolley track, the lead trolley of each carrier being equipped with at least one flop dog and the follower trolley thereof being equipped with a cam plate having a fixed abutment extending upwardly therefrom, a chain track spaced above said trolley track, a drive chain suspended from and movable along said chain track equipped with longitudinally spaced pairs of depending lugs, one lug of each pair being pivoted and the other lug of each pair being fixed, the pivoted lug of each pair adapted to engage the flop dog of a load carrier to drive the latter, and the fixed lug of said pair being normally spaced longitudinally behind the abutment of the cam plate on the follower trolley of said carrier but adapted to engage and to push against said abutment upon disengagement of said flop dog by said pivoted lug on occasions as by inadvertence or by movement of said lead trolley through a switch onto a second or spur trolley track, the vertical spacing between said trolley track and said chain track normally permitting butting engagement between said pivoted lug and said flop dog and between said fixed lug and said abutment, but said tracks adapted to be spaced relatively farther apart in localized sections of the conveyor system as for close packing whereby to position said fixed lug at a level above said abutment to prevent relative engagement therebetween without affecting engagability of said pivoted lug with said flop dog.
 2. The power and free conveyor system set forth in claim 1 wherein the cam plate of each load carrier has a rearwardly opening sLot and a ramp at the open end of said slot, and wherein the pivoted lugs extend downwardly and rearwardly from said drive chain, the flop dog engaging ends of said pivoted lugs are relatively wider than said flop dogs, the flop dog of each load carrier is adapted to enter the slot in the cam plate of a preceding load carrier and the ramp on the cam plate of said preceding load carrier is engageable by entry of said flop dog with the flop-dog-engaging-end of the pivoted lug associated with said flop dog, whereby to disengage said lug from said flop dog.
 3. The power and free conveyor system as set forth in claim 2 wherein said flop dogs and said pivoted lugs have terminal portions disposed to over-and under-lap each other when said drive lugs are in engagement with said flop dogs.
 4. The power and free conveyor system set forth in claim 1 wherein said fixed lugs and said abutments have mutually engageable convex surfaces.
 5. The power and free conveyor system set forth in claim 4 wherein one of said mutually engageable convex surfaces is relatively substantially wider than the other.
 6. The power and free conveyor system set forth in claim 4 wherein the convex surfaces of said fixed lugs are relatively substantially wider than the convex surfaces of said abutments.
 7. A power and free conveyor system having movable work carriers provided with coupled front and rear trolley members, and associated power means having a plurality of pairs of spaced drive lugs, one drive lug of each pair being relatively long and adapted to cooperate with said front trolley to move said work carrier, and the other drive lug of each pair being relatively short and adapted to cooperate with the rear trolley to move said carrier, one portion of said power means being spaced relatively close to said work carriers and another portion of said power means being spaced relatively far from said work carriers, said other relatively short drive lug being disposed to drivingly engage said rear trolley at said one portion of said power means and disposed to be not engageable with said rear trolley at said other portion of said power means, said one relatively long drive lug being disposed to drivingly engage said front trolley at both portions of said power means and being provided with mounting means which is operative to adjust said lug automatically to compensate for variations in the relative spacing between said power means and said work carriers and to maintain said lug positioned for operative engagement with said work carriers under all such variations in spacing. 